Overpotential
The extra voltage beyond the theoretical value that must be applied before an electrode reaction proceeds at a useful rate.
Formula / equation
η = E_applied − E_equilibrium
Unit
V
In depth
Overpotential arises from slow electron transfer kinetics, from the energy needed to form gas bubbles, and from concentration depletion at the electrode surface. Gas evolution has particularly large overpotentials, which is why electrolysis products often defy simple thermodynamic prediction.
Examples in the real world
The high overpotential for oxygen on graphite lets chlorine be produced from brine instead.